Modern diesel fuels are typically formulated with low sulphur levels, often 10 ppmw or less, in order to reduce the pollution caused by their combustion. However, the processes used to remove sulphur-containing components also typically reduce fuel lubricity. It is therefore generally necessary to incorporate lubricity enhancing additives in diesel fuels, in particular to reduce wear on the fuel pumps through which the fuels are conveyed.
It is also necessary, both in the interests of the environment and to comply with increasingly stringent regulatory demands, to increase the amount of biofuels used in automotive diesel fuels. Biofuels are combustible fuels, typically derived from biological sources, which result in a reduction in “well-to-wheels” (ie from source to combustion) greenhouse gas emissions. For use in diesel engines, fatty acid methyl esters (FAMEs) such as rapeseed methyl ester, soybean methyl ester and palm oil methyl ester are the biofuels most commonly blended with conventional diesel fuel components.
However, FAMEs and their oxidation products tend to accumulate in engine oil, which has typically limited their use to 10% v/v or less in fuels burned in many diesel engines. At higher concentrations they can also cause fouling of fuel injectors. Moreover, due to the incomplete esterification of oils (triglycerides) during their manufacture, FAMEs can contain trace amounts of glycerides which on cooling can crystallise out before the FAMEs themselves, causing fuel filter blockages and compromising the cold weather operability of fuel formulations containing FAMEs.
It would be desirable to provide new biofuel-containing diesel fuel formulations which could overcome or at least mitigate the above problems, and which ideally could help to overcome lubricity issues in diesel fuels.